1. Field of the Invention
The present invention relates to an electronic device in which a lead-free (hereinafter referred to as “Pb-free”) metal thin film containing tin (hereinafter referred to as “Sn”) as a main component is formed on the surfaces of external terminals.
2. Description of the Prior Art
Electronic apparatuses which are used in a wide variety of fields are assembled of various electronic devices such as a semiconductor integrated circuit (hereinafter referred to as “IC”), a transistor, a capacitor, a resistor and an inductor. To assemble such electronic apparatuses, a circuit board on which a circuit pattern formed by a electrical conducting layer is printed in advance is used, and a plurality of electronic devices are mounted on the circuit board. More specifically, the external terminals of the electronic device are electrically connected and physically attached to a portion of the circuit pattern with a low-melting solder. To secure reliability of connection between the electronic device and the circuit board, in a metal thin film, which is made generally of a Sn—Pb alloy is formed in advance on the surface of the external terminals of the electronic device by a surface treatment method such as electroplating. (Hereinafter, “reliability of connection between the electronic device and the circuit board” is referred to as “bonding reliability”.)
However, Pb presents a danger to public health and causes environmental pollution when a used electronic apparatus is discarded. Thus, a use of the Sn—Pb alloy is not desirable from the viewpoint of environmental protection. Under such circumstances, a material comprising a Sn-based alloy containing no Pb, which is the so-called Pb-free Sn-based alloy, is required to be used as the low-melting solder. It is also required that a metal thin film comprising the Pb-free Sn-based alloy is formed on the surface of a lead base material of an electronic device by plating.
When the metal thin film comprising the Pb-free Sn-based alloy is formed on the surface of the base member by plating, it is important that the metal thin film, which does not impair the wettability of the low-melting solder and can secure bonding reliability, is formed regardless of which metal is chosen as an additive metal to be added to Sn. As an example, an electronic device in which a Sn-bismuth (hereinafter referred to as “Sn—Bi”) alloy is plated on the surface of a base member as a metal thin film is widely known. As in the case of Pb in the above Sn—Pb alloy, Bi is a metal which forms a low-melting alloy together with Sn and lowers the melting point of the alloy. Applying the Sn—Bi alloy to the metal thin film of external terminals is disclosed in Japanese Patent Laid-Open Publication Nos. 2000-174191, 2001-257303, Hei 11-330340, 2001-53211, 2002-151838, 2002-141456 and Hei 11-251503, and U.S. Pat. No. 6,195,248, U.S. Pat. No. 6,392,293 and U.S. Pat. No. 6,395,583, and U.S. Patent Application Publication No. US 2002/0019077.
When a Sn—Bi alloy is used as the above Pb-free Sn-based alloy and a metal thin film comprising the Sn—Bi alloy is formed on the surface of a base member by plating, the growth of fine metal whiskers is liable to occur on the surface of the external terminal when an electronic device is subjected to an acceleration test such as a temperature cycling test after production of the electronic device, as compared with a case where the metal thin film is formed by use of an Sn—Pb alloy. Further, it is concerned that these whiskers may make short-circuit between the adjacent external terminals. Such short-circuit is more likely to take place in a semiconductor device such as an IC in which a plurality of external terminals are derived from the periphery of a package body at minute intervals. In addition, since the Sn—Bi alloy has poor ductility, bending cracks (hereinafter simply referred to as “cracks”) are liable to occur in the Sn—Bi alloy layer when the external terminals are bent upon, e.g., implementation of the electronic device. Therefore, suppression of the occurrences of whiskers growth and cracks formation is significantly important when a metal thin film comprising a Pb-free Sn-based alloy is formed on the surface of a base member of an electronic device by plating.
This discussion for suppressing the occurrences of whiskers growth and cracks formation in a Pb-free metal thin film, formed on the surface of a base member are made in Japanese Patent Laid-Open Publication Nos. 2000-174191, 2001-257303 and Hei 11-330340.
Japanese Patent Laid-Open Publication No. 2000-174191 discloses a semiconductor device having a lead of which a cross sectional structure is shown in FIG. 12. The lead of the semiconductor device is formed by plating a lower layer 102 which comprises an Sn—Bi alloy having a Bi content of 0.7 wt % (weight %), an intermediate layer 103 which comprises an Sn—Bi alloy having a Bi content of 0.7 to 2.3 wt % and an upper layer 104 which comprises an Sn—Bi alloy having a Bi content of 2.3 wt % on a surface of a base member 101. The three Sn—Bi alloy layers having different Bi contents have such a concentration gradient that the contents of the alloy components increase in a plating film thickness direction.
Further, Japanese Patent Laid-Open Publication No. 2001-257303 discloses a lead material for electronic devices which has a cross sectional structure as shown in FIG. 13. On a surface of a base member 111 of the lead material for electronic devices, a plating layer 112 which comprises an Sn—Cu alloy having a Cu content of 0.4 to 5 wt % and a film thickness of 1 to 15 micrometers (hereinafter referred to as “MIC”) is formed.
Further, Japanese Patent Laid-Open Publication No. Hei 11-330340 discloses a semiconductor device having a lead of which a cross sectional structure is shown in FIG. 14. The lead of the semiconductor device is formed by plating a lower layer 122 which comprises an Sn—Bi alloy having a Bi content of 0 to 1 wt % and a film thickness of 1 to 14 MIC and an upper layer 123 which comprises an Sn—Bi alloy having a Bi content of 1 to 10 wt % and a film thickness of 1 to 12 MIC on a surface of a base member 121.
It is, however, recognized by present inventor that the above prior arts have the following problems.
The conventional semiconductor devices use the Pb-free Sn-based alloys as the metal thin films to be formed on the surfaces of the base members, and the metal thin films are formed by multilayer plating so as to suppress the occurrences of whiskers growth and cracks formation, so that a plating step becomes complicated and costs of a plating process is driven up.
Further, along with the multilayer plating structures, strict control of the contents of the metals to be added to Sn becomes difficult.
In addition, since the contents of the metals to be added to Sn and the plating film thicknesses are not adjusted to right values in accordance with the kinds of the metals to be added to Sn, it is difficult to sufficiently suppress the occurrences of whiskers growth and cracks formation.
For example, it is known that when the above Sn—Bi alloy is plated as a metal thin film to be formed on the surface of a base member of an external terminal, reliability of connection between the external terminal and a circuit board after the electronic device is mounted on the circuit board by soldering is significantly influenced by the content of Bi in the metal thin film. Therefore, it is an important point for guaranteeing product quality to strictly control the Bi content in the metal thin film. For that purpose, the content of Bi in the metal thin film is estimated with high accuracy in a non-destructive manner by a method such as a fluorescent X-ray analysis.